1. Field of the Invention
The present invention relates to composite materials based on boron carbide, titanium diboride and elemental carbon and processes for the production thereof.
2. The Prior Art
Boron carbide is a material which has high hardness and a high resistance to abrasive wear, and is therefore widely used in applications where high abrasion resistance is required, for example for sand blasting nozzles. However, boron carbide has the disadvantage of high brittleness. It is known that pure boron carbide can be pressurelessly sintered to densities of greater than 95% of the theoretical density (TD) of the material only through addition of carbon-containing sintering aids. Free carbon is therefore an unavoidable constituent of the microstructure in all pressurelessly sintered, monolithic boron carbide ceramics. This constituent of the microstructure has a similar effect to finely distributed porosity and therefore negatively influences the mechanical properties of the sintered body, in particular the hardness. It is therefore undesired.
Boron carbide materials containing less than 8% by weight of elemental carbon are hereinafter designated as self-bonded boron carbide. At a free carbon content of, for example, 3% by weight, this material has a fracture toughness of about 2.3 MPa.sqroot.m (measured by the bridge method). Self-bonded boron carbide has therefore not hitherto been able to become established in applications in which the resistance to impact wear plays a part. The situation of limited wear resistance to impact wear is first and foremost due to the insufficient toughness of self-bonded boron carbide. Attempts have therefore been made to reinforce this material, like other brittle monolithic ceramics too, by blending with other materials, for example, by dispersion of particulate hard materials. This particle reinforcement is a known method of increasing the toughness of brittle ceramic materials.
Thus, EP 94,591 (corresponding to U.S. Pat. No. 4,524,138) discloses the toughening of boron carbide by the addition of particles of suitable hard material phases. This disclosure describes polycrystalline sintered bodies of boron carbide with additions of .alpha.-SiC and free carbon, which significantly exceed both the toughness and also the strength of pure boron carbide. In a similar way to silicon carbide, other hard materials in equilibrium with boron carbide can also be used to improve the mechanical properties of boron carbide.
Materials which have proven particularly suitable for the purpose of particle reinforcement of boron carbide are borides, specifically the borides of transition elements of the groups IVa to VIa of the Periodic Table. Shaped bodies having improved wear resistance as a result of the combination of boron carbide with a not-readily-fusible metal boride are described in DE 2,451,774 (corresponding to GB 1,479,589). While the particle composite materials mentioned in DE 2,451,774 are formed by mixing of the desired hard material phases and subsequent sintering, processes have also been described in which the desired phase composition of the material is only formed after a suitable reaction of the starting materials prior to sintering.
Such a process for producing a particle-reinforced composite material of boron carbide and boron-rich transition metal borides, in particular diborides, is disclosed in U.S. Pat. No. 2,613,154. This patent describes shaped bodies comprising boron carbide and more than 50% by volume of a boride selected from the group consisting of TiB.sub.2, VB.sub.2, CrB.sub.2, ZrB.sub.2, NbB.sub.2, Mo.sub.2 B.sub.5, W.sub.2 B.sub.5, HfB.sub.2 and TaB.sub.2. In accordance with the reaction EQU (1+x)B.sub.4 C+2Me.fwdarw.xB.sub.4 C+2MeB.sub.2 +C (1)
a suitable metal and boron carbide form a metal boride, a proportion of unreacted boron carbide which appears suitable and free carbon. Even in boride-particle-reinforced boron carbide ceramics, free carbon is viewed as an undesired constituent of the microstructure. U.S. Pat. No. 2,613,154 thus proposes that the proportion by weight of free carbon has to be less than 2% based on the proportion by weight of boron carbide (see claim 1, column 11, lines 46-48). To keep the amount of free carbon during reaction (1) as small as possible, the use of boron-rich boron carbide is recommended. For example, in lines 40-43, column 3, the use of boron carbide having a boron content of 83% by weight is described. This measure is possible because boron carbide possesses a wide homogeneity range which varies between the compositions B.sub.4.33 C and B.sub.10.5 C.
Another process for producing a particle reinforced composite material of boron carbide and suitable borides, in particular diborides, is disclosed in EP 343,873. There, the boron carbide has added to it a suitable titanium-containing substance such as, for example, titanium oxide, titanium nitride, titanium carbide or metallic titanium and, if desired, a carbon- or hydrogen-containing reducing agent which reacts with boron carbide to form titanium diboride and carbon monoxide, for example, in accordance with EQU (1+x)B.sub.4 C+2TiO.sub.2 +3C.fwdarw.xB.sub.4 C+2TiB.sub.2 +4CO (2)
with the formation of the desired boride. After the reaction, sintering forms a dense shaped body of boron carbide and titanium diboride.
A further process for producing boride-containing boron carbide materials, in which the addition of tungsten carbide or titanium carbide results in the formation of the borides W.sub.2 B.sub.5 or TiB.sub.2 by reaction sintering in accordance with the reaction equations EQU B.sub.4 C+2TiC.fwdarw.3C+2TiB.sub.2 ( 3) EQU 5B.sub.4 C+8WC.fwdarw.8C+4W.sub.2 B.sub.5 ( 4)
is described in U.S. Pat. No. 4,670,408. In this disclosure as well (see column 2, lines 35 to 39), it is recommended that the free carbon which is formed in accordance with the reaction equations (3) and (4) be bound by the addition of elemental boron in the form of boron carbide by the reaction EQU C+4B.fwdarw.B.sub.4 C (5)
to avoid its deleterious effect.
H. Hofmann and G. Petzow have also established (Journal of the Less-Common Metals, Volume 117, pp. 121-127, 1986) that optimal strength is only achieved if boron carbide ceramics reinforced with W.sub.2 B.sub.5 and TiB.sub.2 no longer contain any free carbon.
These documents and also numerous other publications (see, for example, R Telle and G Petzow, "Proceedings of the 9th Int. Symposium on Boron, Borides and Related Compounds," Duisburg 1987, pp. 234-244, and R. Telle, G., Petzow, J. Adlerborn and K. Weiss, ibid., pp. 453-54) agree that boride particles significantly improve the sinterability of pure boron carbide and also its strength and toughness, but that free carbon in the sintered bodies is preferably to be avoided. Thus, free carbon in boride-reinforced boron carbide materials has hitherto been believed to be undesirable and deleterious to the properties of these materials, and for this reason free carbon has been avoided if possible in the specified composite materials.